A biomarker is very useful for diagnosing and monitoring the progress of a disease, and is important for screening patients, monitoring side effects, aiding for the selection of an appropriate treatment and discovering a new drug. In Alzheimer's disease, Aβ (amyloid beta) 1-42 and phosphorylated tau protein in the cerebrospinal fluid are currently considered to be the most useful biomarkers (Non Patent Literatures 1 and 2).